Graceful Restart And High Availability; Low Memory Handling - Cisco Nexus 7000 Series Configuration Manual

Information About Advanced BGP
S e n d d o c u m e n t c o m m e n t s t o n e x u s 7 k - d o c f e e d b a c k @ c i s c o . c o m .
Use the router address-family and neighbor address-family configuration modes to support
multiprotocol BGP configurations. MP-BGP maintains separate RIBs for each configured address
family, such as a unicast RIB and a multicast RIB for BGP.
A multiprotocol BGP network is backward compatible but BGP peers that do not support multiprotocol
extensions cannot forward routing information, such as address family identifier information, that the
multiprotocol extensions carry.

Graceful Restart and High Availability

Cisco NX-OS supports nonstop forwarding and graceful restart for BGP.
You can use nonstop forwarding (NSF) for BGP to forward data packets along known routes in the
Forward Information Base (FIB) while the BGP routing protocol information is being restored following
a fallover. With NSF, BGP peers do not experience routing flaps. During a fallover, the data traffic is
forwarded through intelligent modules while the standby supervisor becomes active.
If a Cisco NX-OS router experiences a cold reboot, the network does not forward traffic to the router and
removes the router from the network topology. In this scenario, BGP experiences a nongraceful restart
and removes all routes. When Cisco NX-OS applies the startup configuration, BGP reestablishes peering
sessions and relearns the routes.
A Cisco NX-OS router that has dual supervisors can experience a stateful supervisor switchover. Before
the switchover occurs, BGP announces that a graceful restart is starting and that BGP will be unavailable
for some time. During the switchover, BGP uses nonstop forwarding to forward traffic based on the
information in the FIB, and the system is not taken out of the network topology. The router that is
restarted marks these routes from its peers as stale.
When a router detects that a graceful restart operation is in progress, both routers exchange their
topology tables. When the router has route updates from all BGP peers, it removes all the stale routes
and runs the best-path algorithm on the updated routes.
After the switchover, Cisco NX-OS applies the running configuration, and BGP informs the neighbors
that it is operational again.

Low Memory Handling

BGP reacts to low memory for the following conditions:
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Note
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See the
shutdown due to a low memory condition.
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
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Minor alert—BGP does not establish any new eBGP peers. BGP continues to establish new iBGP
peers and confederate peers. Established peers remain, but reset peers will not be re-established.
Severe alert—BGP shuts down select established eBGP peers every two minutes until the memory
alert becomes minor. For each eBGP peer, BGP calculates the ratio of total number of paths received
to the number of paths selected as best paths. The peers with the highest ratio are selected to be shut
down to reduce memory usage. You must clear a shutdown eBGP peer before you can bring the
eBGP peer back up to avoid oscillation.
You can exempt important eBGP peers from this selection process.
Critical alert—BGP gracefully shuts down all the established peers. You must clear a shutdown BGP
peer before you can bring the BGP peer back up.
"Tuning BGP" section on page 11-36
Chapter 11
for more information on how to exempt a BGP peer from
Configuring Advanced BGP
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